The present invention relates to ink jet printers in which ink droplets are formed and electrically charged and then deviated to strike a print substrate. It relates to a process intended to mask or reduce misalignment defects and a printer applying this process.
It is known that a pressurized ink jet ejected through a print nozzle can be broken into a series of individual droplets, each droplet being individually charged in a controlled manner. Constant potential electrodes along the path of these individually charged droplets deviate the droplets by a variable amount depending on their charge. If it is not required that a droplet should reach the print substrate, its charge is controlled such that it is deviated to an ink recovery reservoir. The operating principle of this type of ink jet printer is well known, and for example is described in U.S. Pat. No. 4,160,982 A. As described in this patent and as shown in FIG. 1, this type of printer comprises a reservoir 11 containing electrically conducting ink 10 that is distributed through a distribution duct 13 to a droplets generator 16. The role of the droplets generator 16 is to form a set of individual droplets starting from the pressurized ink in the distribution duct 13. These individual droplets are electrically charged by means of a charge electrode 20 powered by a voltage generator 21. The charged droplets pass through a space between two deviation electrodes 23, 24 and are deviated by a variable amount depending on their charge. The least deviated or undeviated droplets are directed to an ink recovery reservoir 22, whereas deviated droplets are directed to a substrate 27. The successive droplets in a burst reaching the substrate 27 can thus be deviated to an extreme low position, an extreme high position and any number of intermediate positions, the set of droplets in the burst forming a vertical line with height xcex94X approximately perpendicular to a relative direction of advance between the print head and the substrate. The print head consists of the droplet generator 16, the charge electrode 20, the deviation electrodes 23, 24 and the recovery reservoir 22. In general, this head is enclosed in a casing not shown. The deviation movement applied to the charged droplets by the deviation electrodes 23, 24 is complemented by a movement along a Y axis perpendicular to the X axis, between the print head and the substrate. The time elapsed between the first and last droplets in a burst is very short. The result is that despite continuous movement between the print head and the substrate, it can be assumed that the substrate has not moved with respect to the print head during the time of a burst. Bursts are fired at regular intervals in space. If all droplets in each burst were directed towards the substrate, then a sequence of lines with height xcex94X would be printed. In general, only some droplets in the burst are directed towards the substrate. Under these conditions, the combination of the relative movement of the head and the substrate, and the selection of the droplets in each burst that are directed towards the substrate, is a means of printing any pattern such as that shown in 28 in FIG. 1. If the line that is drawn with the droplets in a burst is in a direction X, the relative movement of the head and the substrate in the plane of the substrate is in a direction Y perpendicular to X. The undeviated droplets are directed to the recovery reservoir along a path Z perpendicular to the x, y plane of the substrate. Printed droplets reach the substrate by following paths slightly deviated from direction Z.
If the relative movement of the head and the substrate takes place continuously along the largest dimensions of the substrate, there will usually be several print heads printing bands parallel to each other. One example of this type of use is shown in FIGS. 1 and 2 in the patent issued to IBM, as number FR 2 198 410.
If the relative movement of the print head and the substrate in the Y direction takes place along the smallest dimension of the substrate, printing is done band by band, with the substrate performing an intermittent advance movement in the X direction after each scanning. The relative movement of the print head and the substrate is called the xe2x80x9cscanning movementxe2x80x9d. The scanning movement is thus composed of a forward and return movement between a first edge of the substrate and a second edge of the substrate. The movement between one edge and the other edge of the substrate is a means of printing a band of height L, or frequently a part of the band of height xcex94X where xcex94X is usually a sub-multiple of L, without stopping. All bands printed in sequence thus form the pattern to be printed on the substrate. Each time that a band or a part of band is printed, the substrate is advanced by the distance between two bands or parts of bands to print the next band or part of band. Printing may be done during the forward movement only, or during the forward and return movements of the print head with respect to the substrate.
When the pattern to be printed is colored, the different shades of colors are the result of ink impacts from nozzles supplied by inks of different colors being superimposed and placed adjacent to each other. The system for relative displacement of the substrate with respect to the print heads is achieved such that a given point on the substrate is presented in sequence under each of the different colored ink jets. Usually, the print system comprises several jets of the same ink operating simultaneously, either by multiple heads being adjacent to each other or by the use of multi-jet heads, or finally by the combination of these two types of heads in order to achieve high print speeds. In this case, each ink jet prints a limited part of the substrate. The droplets may be produced continuously as described above in relation to FIG. 1. They may also be produced xe2x80x9con demandxe2x80x9d, in other words only when they are necessary for printing needs. In this case, a system for recovery of unused ink is not necessary. Known means of controlling the different jets will now be described with reference to FIG. 2.
The pattern to be printed is described by a numeric file. This file may be formed using a scanner, a calculator aided graphic creation pallet (CAD) transmitted using a calculator data exchange network, or it may simply be read from a peripheral reading a numeric data storage medium (optical disk, CD-ROM). The numeric file representing the colored pattern to be printed is firstly split into several binary patterns (or bitmaps) for each ink. Note that the case of the binary pattern is a non-limitative example; in some printers, the pattern to be printed is of the xe2x80x9ccontonexe2x80x9d type, in other words each position may be printed by a variable number of droplets from 1 to M. Part of the binary pattern is extracted from the file for each jet corresponding to the width of the band that will be printed. FIG. 2, which shows the control electronics of a jet, shows a memory 1 in which the numeric pattern cut into bands is stored, this storage memory containing information about a color. For printing each band, an intermediate memory 2 contains the data necessary for printing the band with the said color. Descriptive data for the band to be printed are then input into a calculator 3 that calculates the charge voltages of the different drops that will form the band for this color. These data are input into the calculator in the form of a sequence of frame descriptions that, when combined, will form the band. The calculator 3 that calculates droplet charge voltages is often in the form of a dedicated integrated circuit. This calculator 3 calculates the sequence of voltages to be applied to the charge electrodes 20, in real time, in order to print a given frame defined by its frame description, as loaded from the intermediate memory 2. An output side electronic circuit 4, called the xe2x80x9cdroplet charge sequencerxe2x80x9d, synchronizes the charge voltages firstly with the times at which droplets are formed, and secondly with the relative advance of the print head and the substrate. The advance of the substrate with respect to the print head is materialized by a frame clock 5, the signal of which is derived from the signal from an incremental encoder of the position of the print unit relative to the substrate. The droplet charge sequencer 4 also receives a signal from a droplet clock 6. This droplet clock is synchronous with the droplet generator control signal 16. It is used to define transition instants of the various charge voltages applied to droplets to differentiate their paths. Numeric data originating from the droplets charge sequencer 4 are converted into an analog value by a digital analog converter 8. This converter outputs a low voltage level and usually requires the presence of a high voltage amplifier 21 that will power the charge electrodes 20. The illustrations of prior art given with reference to FIGS. 1 and 2 are intended to make the domain and benefits of the invention clear, but obviously prior art is not limited to the descriptions made with reference to these Figures. Other arrangements of electrodes and recovery reservoirs for unused ink droplets are described in a very extensive literature. An electromechanical arrangement of charge electrode print nozzles and deviation electrodes as described in invention patent number FR 2 198 410 issued to International Business Machine Corporation (IBM) with reference to FIGS. 1 to 3 in this patent could very well be used in this invention. Similarly, the electronic control circuit for the charge electrodes could be illustrated by the circuit described with relation to FIG. 4 in the same patent. Also, data to be printed need not necessarily be in the form of binary files, but they could be in the form of files containing words of several bits, to translate the fact that each position of the substrate may receive several ink droplets of the same color. It can be understood that for printing, and particularly for color printing, the necessary superposition of droplets originating from different nozzles outputting the different ink colors must be very precise. The main print defects that are generated by all known print systems are related to misalignments along the direction of the relative movement between the print head and the substrate. This defect appears as light or dark lines produced when printing in successive scans. These defects may appear in the space between two bands that in principle must be equal to the interval between two adjacent droplets in a single frame, or within a single band, in the space delimiting the areas printed by different jets, or even inside the frame printed by a jet at the space between two adjacent droplets in the frame. These misalignment defects may be caused either by defects specific to some jets in the print head (mechanical or electrical defects) or substrate positioning errors, or errors of the relative positioning between different print heads, or even between jets in the same print head. Various solutions have been proposed to limit or to eliminate misalignment problems, but all these solutions limit the print rate to a value below the nominal print rate, sometimes by a very high factor, or by redundant print heads and therefore at high cost. Some examples of frequently used known solutions for limiting misalignment will be described very briefly below; a first type of solution is based on fine mechanical adjustments of the positions of print heads by means of micrometric tables. This solution is expensive due to the necessary number of micrometric tables, and frequently painstaking due to the number of trial and error attempts that are necessary.
Another frequently used type of solution consists of using a very high overlap ratio between adjacent drops, in order to avoid white misalignments. These white misalignments correspond to the lack of coverage of the substrate. Dark misalignments are less easily seen and it is preferred to have a misalignment defect composed of dark lines rather than a misalignment defect composed of white lines. The solution consisting of increasing the overlap ratio between adjacent droplets is efficient to compensate for defects within a single band and to a certain extent misalignment defects between bands, but it has the disadvantage that it requires a very large quantity of ink per unit area of substrate and causes difficulties in drying or deformation of the substrate.
A third type of solution for eliminating misalignment defects on printers operating in scanning consists of printing the substrate partially during each scanning. The substrate is completely covered by increasing the number of times that the substrate is scanned. Printing in several passes in this way uses several strategies for interlacing the positions of droplets from different jets. One example of interlacing even and odd lines is given in patent number U.S. Pat No. 4,604,631 issued to the RICOH Company. One advantage of this solution, frequently related to a high overlap ratio, is that it enables a substrate drying time, but it reduces the print rate by a factor of between 2 and 16.
The performances of colored graphic print systems are naturally moving towards higher and higher resolutions and rates, consequently there is an increasingly critical need to efficiently limit misalignment problems without making compromises that reduce print rates.
The process according to the invention is intended to mask some misalignment problems without modifying the print speed.
This invention does not necessitate a high droplet overlap ratio. It can achieve high print rates with a relatively small number of print heads. When the overlap between adjacent droplets is minimized, a misalignment defect can remain, and particularly a white misalignment defect that appears regularly. This defect is very perceptible to the naked eye when it is regular. The perceptibility of a defect of this type can be reduced by superposing an additional noise voltage onto the nominal droplet charge voltage, in order to vary the nominal position of each droplet so that there is a random dispersion in its real position. Due to this dispersion in the real position of each droplet around its nominal position, the misalignment defect no longer appears as a continuous straight line and therefore becomes less perceptible to the naked eye.
Therefore, the invention relates to a process for modification of the position at which electrically-charged ink droplets arrive on a substrate, in an adjustable and sequential manner by charge electrodes, the droplets originating from a print head, deviation electrodes being provided to modify the paths of the droplets between N nominal positions from a first position X1, a last position XN, and Nxe2x88x922 intermediate positions, the N positions defining a frame in the form of a straight segment parallel to an X direction of the substrate, process characterized in that a nominal voltage is applied to the droplet charge electrodes, superposed with an additional random algebraic voltage thus masking a misalignment fault by dispersion of the real position of each droplet around its nominal position.
The average amplitude of this noise voltage will depend on the row j of the droplet in the frame. Preferably, the maximum amplitude of the additional noise voltage will be equal to a fraction less than 1 of the smallest difference between the nominal voltage Vj to be applied to the row j droplet and the nominal voltage Vj+1 or Vjxe2x88x921 to be applied to one of the droplets immediately adjacent to the row j droplet in the printed frame, in other words row j+1 and row jxe2x88x921 droplets.
Since the values of the differences in the charge voltages applied to adjacent printed droplets are fairly similar, the maximum value of the random additional voltage could be assumed to be a fraction of an average value, this average value being the average value of differences in nominal voltages between two adjacent droplets printed in the frame.
Preferably, the minimum amplitude of the additional noise voltage will be equal to the value of the voltage difference that can be obtained by varying the value of the least order bit of an analog digital converter, the output of which supplies a high voltage amplifier coupled to the droplet charge electrodes.
Preferably, the amplitude of the additional noise voltage will correspond to a random numeric value generated by a pseudo-random number generating algorithm. The correspondence between the random numeric value and the additional noise voltage will be a result of the application of this numeric value to the digital analog converter. The regular dark or light misalignment defect will no longer appear, or will be less obvious.
The invention also relates to a printer provided with means of performing the process according to the invention, in this case a printer with a continuous deviated jet projecting droplets in rows 1 to N in bursts, the droplets in a burst being directed or not directed towards a print substrate depending on data that define a pattern to be printed, the printer having at least:
a print head, this head comprising associated means of separating at least one ink jet into droplets and a charge electrode for droplets, means of deviating some of the droplets towards the print substrate,
means of controlling the printout including a means of injecting the charge into the droplets to be directed towards the substrate depending on their row in the burst, coupled with the droplet charge electrode,
characterized in that the means of controlling the printout comprise a random additional voltage generator coupled to droplet charge injection means, the droplet charge injection means taking account of the value of the random voltage generated by the additional random voltage generator to modify the charge voltage of each droplet as a function of the generated random value, the droplets of each row thus being dispersed around a central position corresponding to the position that they would have had without any additional voltage.
In the preferred embodiment of the invention for a printer operating by scanning, the printer also comprises a position detector detecting the position of a mark printed before each first frame of a band, this detector outputting a value representative of a variation between the real and nominal positions of the substrate, and in that the print control means also comprise a calculator that calculates the dynamic translation correction voltage xcfx86 for the substrate advance, this calculator determining a dynamic translation correction voltage xcfx86 for the substrate advance for each droplet in a burst as a function of its row, this correction voltage taking account of a value of a variation in the advance of the substrate output by means coupled to the detector and calculating a value of the difference from a nominal position, the calculator that calculates the dynamic translation correction voltage xcfx86 for the substrate advance being coupled to means of injecting the droplet charge, the means of injecting the droplet charge taking account of the value of the substrate advance correction voltage generated by the calculator that calculates the dynamic translation correction voltage xcfx86 for the substrate advance to modify the charge voltage on each droplet as a function of the dynamic translation correction voltage xcfx86 for the substrate advance.